1. Field of the Invention
The present invention relates to the field of the digital transmissions of data and more particularly to a synchronization and equalization device for a receiver in a digital transmission system.
2. Description of the Related Art
Any person qualified in the art having to design a receiver for a digital transmission system, a receiver intended to receive either a signal in baseband or the output of a complex demodulator in the case of modulation-demodulation, will typically be confronted with two major problems which are symbol synchronization and equalization.
The first problem is that of symbol synchronization, which is also known as timing recovery (or also clock recovery in specialized literature). Symbol synchronization consists in allowing the receiver to extract from the baseband received signal—or even from the received signal's complex envelope in the case of modulation demodulation—the clock which was used on emission to insert the sequence of digital symbols a(k) carrying information. FIG. 1 shows traditional modeling of a digital communication chain in which are provided a transmitter modeled by a filter 11 associated by Fourier transform with an impulse response gTe(t) corresponding to shaping of the sequence of symbols a(k) at emission and to its transformation into a continuous waveform, a data communication channel 12 also associated by Fourier transform with an impulse response c(t) and which further adds some noise w(t). For the receiving part, are provided a sampler 13 controlled by a sampling clock with a Ts period, a filter type equalizer 14 and a filter type equalizer control device 15. As can be seen, once locally rebuilt, the reception clock is used to control extraction of received signal r(t) samples at a rate (k×TS), which must be synchronous with symbol times at transmission.
Many techniques exist for carrying out such symbol synchronization in order to obtain a reception clock which provides the optimum moments for the received signal sampling. However, all these techniques have a major defect, which results from their sensitivity to the data communication channel c(t). Indeed, the techniques which are usually employed are based on the assumption that the data communication channel's influence is negligible or—hardly better—that signals are correctly equalized. Even if it is generally possible to know the emission impulse response gTe(t) (represented by 11 in FIG. 1) with a certain precision and therefore to deduce a matched filter g*Te(−t) before sampling by element 13, the precise value of the data communication channel impulse response remains undetermined and undeterminable before sampling by sampler 13. FIG. 1 illustrates the classically recommended structure for a receiver which comprises such a preliminary processing before sampling of the reception signal. Matched filtering is applied by means of a filter 16 with an impulse response g*Te(−t) and which thus realizes processing adapted to the emission shaping. This filtering is then followed with synchronous sampling by sampler 13, then with optimal digital processing by means of digital filter 14. As can be seen, if the effect of emission shaping can thus be compensated, however the effect of the propagation medium cannot be taken into account nor dealt with, this effect tends to become increasingly important as this medium is submitted to higher and higher frequencies. With today's commonly used transmission rates, the effect of the physical environment of propagation can no longer be regarded as negligible so that the sampler 13 driving device is disturbed by the convolution effect resulting from the transfer function of the channel of propagation.
The second problem to which the designer of a reception system is confronted is equalization; i.e., taking into account the data communication channel which has already been described as having undetermined and variable responses. When transmission rates increase, symbol time duration becomes short relative to the length of the data communication channel impulse response. This means that, at each precise instant, the received signal by the receiver depends on several consecutive symbols. The effect of the channel tends to introduce a convolution between the various emitted symbols. Generally, the effect of the data communication channel is compensated for by carrying out equalization by means of a digital filter 14 which is introduced after the sampler, as can be seen in FIGS. 1 and 2. Parameters or internal coefficients of this digital equalization filter are adjusted so as to compensate as well as possible for the effect introduced by the transmission medium. In some known techniques, parameters are adjusted through a preliminary learning phase in which transmission-reception of a sequence of symbols known by the receiver are carried out, so that the receiver may correctly adjust its equalization parameters. In other techniques, adaptive digital algorithms are used which operate <<blindly>>, i.e., which do not use such a preliminary learning phase.
Classically, known equalization techniques, and the previously mentioned synchronization techniques, operate independently from one another. Clearly, the designer of a synchronization system usually starts from the postulate that the signal which is received and from which samples are to be synchronously taken at judiciously selected moments comes from a perfect or correctly equalized channel. On the other hand, any person willing to design an equalization device will start from a sampling operation supposed to be synchronous with emission, i.e., perfectly matching with emission symbol times, so as to ensure convergence of the adaptation process of his/her digital equalization filter.
As can be seen, the traditional approach has crippling limitations as it is not possible to perfectly solve any of these problems independently from the other. It is desirable to completely rethink the architecture of a transmission system receiver to commonly and simultaneously process correctly the received signal and, to consequently address the problem of symbol synchronization and the problem of channel equalization.